Process for the preparation of hydroxylammonium

ABSTRACT

The invention relates to a process for the preparation of hydroxylammonium, said process comprising the steps of:
     a) feeding gaseous hydrogen to a reaction mixture, said reaction mixture comprising an aqueous reaction medium and a gaseous phase;   b) catalytically reducing, in said reaction mixture, nitrate or nitrogen oxide with hydrogen to form the hydroxylammonium;   c) withdrawing a gas mixture from the reaction mixture, said gas mixture comprising gaseous hydrogen and gaseous non-hydrogen compounds;   d) separating at least part of the gaseous non-hydrogen compounds from the gas mixture to obtain a hydrogen-enriched gas; and   e) passing the hydrogen-enriched gas to a hydrogenation zone.

CROSS REFERENCE TO RELATED APPLICATION

This application is the National Phase of International ApplicationPCT/NL02/00460 filed Jul. 11, 2002 which designated the U.S., and thatInternational Application was published under PCT Article 21(2) inEnglish.

The invention relates to a process for the preparation ofhydroxylammonium by catalytically reducing nitrate with hydrogen.

Hydroxylammonium can be formed by reducing nitrate with hydrogen. Thereaction can be effected in aqueous reaction medium in the presence of acatalyst, e.g. palladium and/or platinum on a carrier. The reduction ofnitrate can be represented as follows:2H⁺+NO₃ ⁻+3H₂ - - - →NH₃OH⁺+2H₂O

EP-A-773189 and WO-A-9818717 describe a process for the production ofhydroxylammonium by catalytic reduction of nitrate with hydrogen.Gaseous hydrogen is fed to a reaction mixture in which the reduction iseffected. An off-gas is continuously withdrawn from the reactionmixture, and analyzed, the off-gas including H₂, N₂, NO and N₂O.

NL-A-6908934 describes a process for the production of hydroxylammoniumwherein gaseous hydrogen is continuously fed into a bubble column, inwhich nitrate is catalytically reduced in a reaction mixture comprisingan aqueous reaction medium and a gaseous phase. A gas mixture iswithdrawn from the bubble column. The gas mixture contains gaseoushydrogen and gaseous non-hydrogen compounds, such as for instanceby-products of the reaction or inerts which may be fed to the reactionmixture together with the hydrogen. It is described that the gas mixturewithdrawn is recycled into the bubble column or purged in whole or inpart. In practice, such purge is used to operate the process for anextended period of time.

Disadvantage of the process of NL-A-6908934 is that either theefficiency, i.e. the molar quantity of desired product obtained permolar quantity of hydrogen fed to the reaction zone, is relatively low,or the activity, i.e. the molar quantity of hydroxylammonium formed perhour per kilogram of catalyst and the selectivity, i.e. the molarquantity of hydroxylammonium formed per mol of converted startingproduct, are relatively low.

We observed that a relatively low efficiency is obtained when the purgestream is relatively large and that the activity and selectivity arerelatively low for a relatively small purge stream.

Goal of the invention is to provide a process, wherein a highefficiency, activity and selectivity can be combined.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying FIGURE is a schematic flow diagram illustrating anembodiment of a system for carrying out an embodiment of the presentinvention.

This goal is achieved according to the invention by providing a processfor the preparation of hydroxylammonium, said process comprising:

-   a) feeding gaseous hydrogen to a reaction mixture, said reaction    mixture comprising an aqueous reaction medium and a gaseous phase;-   b) catalytically reducing, in said reaction mixture, nitrate with    hydrogen to form the hydroxylammonium;-   c) withdrawing a gas mixture from the reaction mixture, said gas    mixture comprising gaseous hydrogen and gaseous non-hydrogen    compounds; characterized in that the process also comprises:-   d) separating at least part of the gaseous non-hydrogen compounds    from the gas mixture to obtain a hydrogen-enriched gas; and-   e) passing the hydrogen-enriched gas to a hydrogenation zone.

According to one aspect of the invention the efficiency is increasedwith no or only limited decrease of the activity and/or selectivity.According to another aspect of the invention the activity and/orselectivity are increased with no or only limited decrease ofefficiency. According to a further aspect of the invention the activityand/or selectivity and the efficiency are increased.

The hydrogen-enriched gas may be passed to any hydrogenation zone inwhich a hydrogenation reaction is effected. Examples of hydrogenationreactions are hydrogenation of benzene and hydrogenation of phenol.Preferably, the hydrogen-enriched gas is passed to a hydrogenation zonein a production process for caprolactam and/or for intermediatesthereof, such as for instance hydrogenation of benzene, hydrogenation ofphenol and/or reduction of nitrate or nitrogen oxide. Most preferably,the hydrogen-enriched gas is passed to the reaction mixture. It ispossible to pass the hydrogen-enriched gas directly to the reactionmixture. It is also possible to combine the hydrogen-enriched gas firstwith other gas streams which are fed to the reaction mixture.

The separation is not limited to a specific separation technique.Membrane separation, adsorption techniques, including for instancepressure swing adsorption, and cryogene distillation may be used. Theseparation may be effected using membrane separation. Any suitablemembrane may be used. Suitable membranes include membranes having ahigher permeability for gaseous H₂ than for the gaseous non-hydrogencompounds to be separated. Membranes made from polyimide may be used.Membrane separation is for instance described in “Membrane SeparationsTechnology, Principles and Applications”, R. D. Noble and S. A. Stemeds., Elsevier Science B. V, The Netherlands (1995) ISBN 0-444-81633-X,pp. 632–644 and “Handbook of Industrial Membrane Technology”, M. C.Porter, ed., Noyes Publications, New Jersey, USA (1988) ISBN0-8155-1205-8, pp. 579–588. Adsorption, preferably pressure swingadsorption, is advantageously used when the hydrogen-enriched gas ispassed into the reaction mixture. This has the advantage that recyclingmay be effected with no or only limited re-pressurizing of thehydrogen-enriched gas after separation. Adsorption techniques are forinstance described in “Gas Separation by Adsorption Processes”, by R. T.Yang, Imperial College Press, UK (1999) ISBN 1-86094-047-1, pp. 255–260.The separation may be carried out by feeding the entire gas mixture or apart of the gas mixture to a separator and by withdrawing thehydrogen-enriched gas from the separator. When feeding only part of thegas mixture to the separator, the part which is not fed to theseparator, is preferably recycled to the reaction mixture.

The gas mixture withdrawn from the reaction zone comprises gaseoushydrogen (H₂) and gaseous non-hydrogen compounds. As used herein,hydrogen refers to H₂, and gaseous non-hydrogen compounds refer togaseous compounds other than H₂. The gaseous non-hydrogen compounds mayfor example include CH₄, H₂O, NO, NO₂, N₂, and/or N₂O. The gaseousnon-hydrogen compounds may for instance include by-products of thereduction (e.g. H₂O, NO, NO₂, N₂, and/or N₂O) and/or compounds which maybe fed to the reaction zone together with the gaseous hydrogen (e.g.CH₄, and/or N₂).

The molar fraction of hydrogen in the gas mixture (with respect to thetotal molar quantity of all gaseous compounds in the gas mixture, i.e.the molar quantity of H₂ in the gas mixture divided by the sum molarquantity of all gaseous compounds in the gas mixture) is not limited toa specific value. A gas mixture may for example withdrawn in which themolar fraction of hydrogen is higher than 0.35, preferably higher than0.4, more preferably higher than 0.5, most preferably higher than 0.6.Increasing the molar fraction of hydrogen in the gas mixture has theadvantage that the hydrogen partial pressure in the reaction mixture isbrought to a higher level (for a constant total pressure). There is nospecific upper limit for the molar fraction of hydrogen in the gasmixture. For practical reasons, the molar fraction of hydrogen in thegas mixture is usually lower than 0.95, in particular lower than 0.9.

The hydrogen partial pressure in the reaction mixture may be higher than0.9 MPa, preferably higher than 1.0 MPa, more preferably higher than 1.3MPa, most preferably higher than 1.5 MPa. An increased hydrogen partialpressure in the reaction mixture has the advantage that the activityand/or selectivity is increased. As used herein the hydrogen partialpressure in the reaction mixture refers to the molar fraction ofhydrogen in the gas mixture multiplied by the total pressure in thereaction mixture. Preferably, the total pressure in the reaction mixtureis higher than 1.5 MPa, more preferably higher than 2.0 MPa, mostpreferably higher than 2.5 MPa. The total pressure in the reactionmixture is preferably lower than 4.0 MPa, more preferably lower than 3.5MPa, in particular lower than 3.0 MPa.

The molar fraction of non-hydrogen compounds in the gas mixture may verybetween wide ranges. The molar fraction of N₂ in the gas mixture may forexample be between 0.02 and 0.65, preferably between 0.05 and 0.5. IfCH₄ is present in the gas mixture, the molar fraction of CH₄ in the gasmixture may for example be between 0 and 0.65, preferably between 0 and0.5. The molar fraction of N₂O in the gas mixture may for example bebetween 0.001 and 0.08, preferably below 0.05, more preferably below0.03. The molar fractions are given with respect to the sum molarquantity of all gaseous compounds in the gas mixture.

According to the invention at least part of the gaseous non-hydrogencompounds are separated from the gas mixture. Gaseous non-hydrogencompounds which are advantageously separated include for instance N₂O,N₂, NO, NO₂, H₂O and/or CH₄.

According to the invention the separation of gaseous non-hydrogencompounds from the gas mixture results in a hydrogen-enriched gas. Themolar fraction of hydrogen in the hydrogen-enriched gas (with respect tothe total molar quantity of all gaseous compounds in thehydrogen-enriched gas. i.e. the molar quantity of H₂ in thehydrogen-enriched gas divided by the sum molar quantity of all gaseouscompounds in the hydrogen-enriched gas) is higher than the molarfraction of hydrogen in the gas mixture (with respect to the total molarquantity of all gaseous compounds in the gas mixture). Preferably, themolar fraction of hydrogen in the hydrogen-enriched gas is at least 0.05higher than the molar fraction of hydrogen in the gas mixture, morepreferably at least 0.1, most preferably 0.2 higher than the molarfraction of hydrogen in the gas mixture. Preferably, the molar fractionof N₂O in the hydrogen-enriched gas is lower than the molar fraction ofN₂O in the gas mixture. Preferably, the molar fraction of N₂ in thehydrogen-enriched gas is lower than the molar fraction of N₂ in the gasmixture. If CH₄ is present in the gas mixture, the molar fraction of CH₄in the hydrogen-enriched gas is preferably lower than the molar fractionof CH₄ in the gas mixture.

The reaction mixture comprises an aqueous reaction medium and a gaseousphase. Typically, the aqueous reaction medium is acidic, the pHpreferably being between 0.5 and 6, more preferably between 1 and 4.Preferably, the aqueous reaction medium is buffered. Preferably, theaqueous reaction medium contains sulfuric acid or phosphoric acid, morepreferably phosphoric acid. Preferably, the phosphate concentration inthe aqueous reaction medium is higher than 2.0 mol/l. Preferably, anaqueous product stream containing the hydroxylammonium formed iswithdrawn from the aqueous product stream medium, the concentrationhydroxylammonium in the aqueous product stream preferably being higherthan 0.8 mol/l. The gaseous phase generally contains hydrogen, andnon-hydrogen compounds, the composition being dependent on the relativeflow rates of the hydrogen and non-hydrogen compounds to and from thereaction mixture and on the rate at which the reduction is effected. Therelative volume of the gaseous phase may vary between wide ranges.Preferably, the volume percentage of the gaseous phase is between 15 to50 vol. % (relative to the volume of aqueous reaction medium plus thevolume of the gaseous phase plus the volume of the catalyst).

The nitrate (NO₃ ⁻) may be reduced at any suitable temperature, forinstance at a temperature ranging from 20 to 100° C., preferably 30–90°C., more preferably 40–65° C. The reaction mixture comprises a catalyst.Preferably, the catalyst comprises a precious metal on a support,preferably platinum, palladium, or a combination of palladium andplatinum on a support. Preferably, the support comprises carbon oralumina support, more preferably carbon. The catalyst employed in thereaction zone preferably comprises between 1 to 25 wt. %, morepreferably between 5 to 15 wt. % of the precious metal, relative tototal weight of support plus catalyst. Preferably, the catalyst furthercomprises an activator. The activator is preferably selected from thegroup consisting of Cu, Ag, Au, Cd, Ga, In, Tl, Ge, Sn, Pb, As, Sb andBl, most preferably Ge. Generally, the catalyst is present in an amountof 0.2–5 wt. % relative to the total liquid weight of the aqueousreaction medium. The reduction may be effected in any suitable reactor,for instance a reactor with a mechanical stirrer or a column, mostpreferably a bubble column. An example of suitable bubble column isdescribed in NL-A-6908934. Preferably, the process according to theinvention is process is a continuous process.

An embodiment of the process according to the invention will now bedescribed with reference to FIG. 1. However the process according to theinvention is not limited to this embodiment.

In FIG. 1, the reaction mixture is present in a reactor A, in this casea bubble column. The aqueous reaction medium and catalyst arecontinuously passed through (A) by circulation via line (6). Thecatalyst comprises a precious metal on a support. A liquid feed stream,containing nitrate is fed to the reaction mixture via line (7) byintroducing it into circulation (6). An aqueous product streamcontaining hydroxylammonium is withdrawn (using a filter unit, notshown) via line (8). A gaseous stream containing gaseous hydrogen is fedto the reaction mixture via line (1). A gas mixture containing hydrogenand non-hydrogen compounds is withdrawn from the reaction mixture vialine (2). Part of the gas mixture is recycled to the reaction mixturevia line (2a). Another part of the gas mixture is passed to separator(B) via line (2b). In separator (B) at least part of the non-hydrogencompounds is separated from the gas mixture via line (3), resulting in ahydrogen-enriched gas. The hydrogen-enriched gas is withdrawn fromseparator (B) via line (4). A gaseous feed stream containing hydrogen tobe converted is supplied via line (5) and combined with thehydrogen-enriched gas (supplied via line (4)) and the gas mixture(circulated via line (2a)) to form the gaseous stream which is fed tothe reaction mixture via line (1). Separator B may include one or moremembrane separation units or a pressure swing absorption unit.

The invention will be further elucidated with reference to the followingexamples. These examples should not be construed as limiting the presentinvention.

REFERENCE EXPERIMENT (A) AND EXAMPLES I–III

In all examples hydroxylammonium is prepared using a set-up as indicatedin FIG. 1 by using membrane separation in reference experiment (A) thesame set-up is used with the exception that the gas mixture withdrawnvia line (2b) is not fed to separator (B), but purged instead withoutbeing recycled to reaction zone (A). In all examples and in thecomparative experiment the aqueous reaction medium and catalyst (10%Palladium on activated carbon, weight percentage given with respect tothe sum weight of palladium+activated carbon. The catalyst was activatedwith 4.5 g GeO₂ per kg of catalyst) is continuously recycled via line(6), the aqueous reaction medium exiting reaction zone (A) containing1.625 mol/l NH₃OH.H₂PO₄, 1.125 mol/l NH₄NO₃, 1.50 mol/l NH₄H₂PO₄ and0.75 mol/l H₃PO₄. Per hour 45 kmol of hydroxylammonium is produced whichis withdrawn via line (8) (flow rate 27.7 m³/hour). The flow rate of theaqueous reaction medium exiting reaction zone (A) via line (6) is afactor 10 higher than the flow rate of the product stream withdrawn vialine (8). The flow rate of the gas mixture circulating via line (2a) isa factor 5.2 higher than that of the gaseous feed stream supplied vialine (5). The temperature in reaction zone A is 54° C., the total in thereaction mixture 2.65 MPa.

The following definitions are used.

-   H₂ partial pressure: molar fraction of hydrogen in the gas mixture    withdrawn via line (2), multiplied by the total pressure.-   NO₃ ⁻ selectivity: moles of hydroxylammonium formed per mol of    converted NO₃ ⁻-   H₂ selectivity: moles of hydroxylammonium formed per 3 moles of    converted H₂-   activity: moles of hydroxylammonium formed per hour per kilogram of    catalyst-   H₂ efficiency: moles of hydroxylammonium formed per 3 moles of H₂    supplied to the reaction zone (via line (5)).

Reference Experiment A

Hydroxylammonium is prepared as described above, whereby the gas mixturewithdrawn via line (2b) is not fed to separator (B), but purged instead.The amount of catalyst used is 567 kg. Per hour 202.3 kmol hydrogen issupplied via line (5). The flow rate of the gas mixture withdrawn vialine (2b) is 36.0 kmol/hr, the molar fraction of hydrogen being 0.461.The composition of the various gas flows are indicated in table 1. Thefollowing results are obtained.

-   H₂ partial pressure: 1.22 MPa-   H₂ selectivity: 72.7%-   activity: 79.4 mol·hr⁻¹·kg⁻¹.-   NO₃ ⁻ selectivity: 75.1%-   Efficiency: 67%

Example I

The process is carried out according to the invention by repeatingreference experiment A, the difference being that the gas mixturewithdrawn via line (2b) is fed to separator (B) and that thehydrogen-enriched gas obtained is recycled to the reaction zone. Thehydrogen supply (via line (5)) is decreased to 193.7 kmol/hr.

The composition of the various gas flows are indicated in table 1. Thefollowing results are obtained.

-   H₂ partial pressure: 1.22 MPa-   H₂ selectivity: 72.7%-   activity: 79.4 mol·hr⁻¹·kg⁻¹.-   NO₃ ⁻ selectivity: 75.1%-   Efficiency: 70%

This example shows that the efficiency is increased from 67% to 70%without decreasing the activity and selectivity.

Example II

The process is carried out according to the invention by repeatingreference experiment A, the difference being that the gas mixturewithdrawn via line (2b) is fed to separator (B) and that thehydrogen-enriched gas obtained is recycled to the reaction zone. Theflow rate of the gas mixture withdrawn via line (2b) is increased from36.0 to 57.1 kmol/hr, the molar fraction of hydrogen being 0.655. Theamount of catalyst is decreased from 567 kg to 509 kg. The compositionof the various gas flows are indicated in table 1. The following resultsare obtained

-   H₂ partial pressure: 1.73 MPa-   H₂ selectivity, 74.4%-   activity: 88.4 mol·hr⁻¹·kg⁻¹.-   NO₃ ⁻ selectivity: 76.5%-   Efficiency: 67%

This example shows that the activity and selectivity is increasedwithout decreasing the efficiency. This has the advantage that lesscatalyst is needed. The increased H₂ selectivity is advantageous, sincefewer by-products are formed. The increased NO₃ ⁻ selectivity isadvantageous, since fewer by-products are formed and since less NO₃ ⁻ isconverted per quantity of hydroxylammonium formed.

Example III

The process is carried out according to the invention by repeatingreference experiment A, the difference being that the gas mixturewithdrawn via line (2b) is fed to separator (B) and that thehydrogen-enriched gas obtained is recycled to the reaction zone. Thesupply of hydrogen is decreased from 202.3 kmol/hr to 198.0 kmol/hr. Theflow rate of the gas mixture withdrawn via line (2b) is increased from36.0 to 50.9 kmol/hr, the molar fraction of hydrogen being 0.617. Theamount of catalyst is decreased from 567 kg to 521 kg. The hydrogensupply (via line (5)) is decreased from 202.3 kmol/hr to 198 kmol/hr.The composition of the various gas flows are indicated in table 1. Thefollowing results are obtained.

-   H₂ partial pressure: 1.63 MPa-   H₂ selectivity: 72.7%-   activity: 86.4 mol·hr⁻¹·kg⁻¹.-   NO₃ ⁻ selectivity: 76.2%-   Efficiency: 68%

This example shows that the efficiency, activity and selectivity areincreased.

TABLE 1 Composition and flow rates in experiment A and examples I–IIIline 5 line 5 lines 2, 2a, 2b line 2b line 4 line 4 line 3 line 3 comp.flow comp. flow comp. flow comp. flow (molar (kmol/ (molar (kmol/ (molar(kmol/ (molar (kmol/ Exp. fraction) hr) fraction) hr) fraction) hr)fraction) hr) A H₂ 0.925 202.3 0.461 16.6 — — — — N₂ 0 0 0.080 2.9 — — —— CH₄ 0.075 16.4 0.456 16.4 — — — — N₂O 0 0 0.004 0.1 — — — — I H₂ 0.925193.6 0.461 16.5 0.938 8.5 0.299 8.0 N₂ 0 0 0.082 2.9 0.008 0.1 0.1072.9 CH₄ 0.075 15.7 0.453 16.2 0.054 0.5 0.589 15.7 N₂O 0 0 0.004 0.1 0 00.005 0.1 II H₂ 0.925 202.3 0.656 37.4 0.977 16.6 0.518 20.8 N₂ 0 00.050 2.8 0.003 0 0.070 2.8 CH₄ 0.075 16.4 0.293 16.7 0.02 0.3 0.40916.4 N₂O 0 0 0.002 0.1 0 0 0.003 0.1 III H₂ 0.925 198.0 0.617 31.4 0.97214.8 0.466 16.6 N₂ 0 0 0.056 2.9 0.004 0.1 0.079 2.8 CH₄ 0.075 16.10.324 16.5 0.024 0.4 0.452 16.1 N₂O 0 0 0.003 0.1 0 0 0.004 0.1

1. Process for the preparation of hydroxylammonium, said processcomprising: a) feeding gaseous hydrogen to a reaction mixture, saidreaction mixture comprising an aqueous reaction medium and a gaseousphase; b) catalytically reducing, in said reaction mixture, nitrate withhydrogen to form the hydroxylammonium ; c) withdrawing a gas mixturefrom the reaction mixture, said gas mixture comprising gaseous hydrogenand gaseous non-hydrogen compounds; d) separating at least part of thegaseous non-hydrogen compounds from the gas mixture, resulting in ahydrogen-enriched gas; and e) passing the hydrogen-enriched gas to ahydrogenation zone.
 2. Process according to claim 1, wherein saidprocess comprises passing the hydrogen-enriched gas to the reactionmixture.
 3. Process according to claim 1, wherein said process comprisesseparating said at least part of the non-hydrogen compounds from saidgas mixture by using a membrane.
 4. Process according to claim 1,wherein said process comprises separating said at least part of thenon-hydrogen compounds from said gas mixture by using pressure swingadsorption.
 5. Process according to claim 1, wherein said processcomprises separating said at least part of the non-hydrogen compoundsfrom said gas mixture by using cryogene distillation.
 6. Processaccording to claim 1, wherein the molar fraction of hydrogen in the gasmixture is higher than 0.4.
 7. Process according to claim 6, wherein themolar fraction of hydrogen in the gas mixture is higher than 0.5. 8.Process according to claim 1, wherein the hydrogen partial pressure inthe reaction mixture is higher than 1.0 MPa.
 9. Process according toclaim 8, wherein the hydrogen partial pressure in the reaction mixtureis higher than 1.3 MPa.
 10. Process according to claim 1, wherein saidat least part of the gaseous non-hydrogen compounds include N₂. 11.Process according to claim 1, wherein said at least part of the gaseousnon-hydrogen compounds include OH₄.
 12. Process according to claim 1,wherein said at least part of the gaseous non-hydrogen compounds includeN₂O.
 13. Process according to claim 1, wherein said process is acontinuous process.